An embodiment of the present invention relates to a method and apparatus for determining the movement of an object in an imager. In particular, an embodiment of the present invention relates to three-dimensional movement of a patient positioned on a table between a radiation source and an image receiver of an imaging apparatus.
In the field of medical imaging, it is well known to use radio-opaque markers positioned on the object, such as a patient, as reference points in order to assist the guiding of surgical instruments during an operation and/or to permit the merging of images, for instance the superposition of images acquired by an imaging apparatus. The imaging apparatus conventionally comprises a means for providing a radiation source, such as X-rays, positioned opposite a means for receiving an image, the means for providing a radiation source and the means for receiving an image being driven in relative rotation about at least one axis, usually three axes, means for operating, means for acquisition, means for visualizing the images and means for control. The patient is positioned on a table that can be moved in the three possible translations associated with a given space, i.e., longitudinally, laterally and vertically, so that the part of the patient's body to be examined and/or treated extends between the means for providing a radiation source and the means for receiving an image. This mobility of the table and the radiation source and the image receiver allows a practitioner to acquire images for any part of the body of a patient lying on the table. For instance, it is customary to use two-dimensional fluoroscopic images obtained by irradiating the patient with low X-ray doses during an intervention in order to guide the instrument in the patient's organ to be treated. The information associated with these fluoroscopic images may preferably be introduced into three-dimensionally reconstructed images in order to improve the guiding of the surgical instruments. Alternatively, acquired three-dimensional images may be projected onto the two-dimensional fluoroscopic images acquired during the intervention.
In order to permit these projections of 3D images onto 2D fluoroscopic images or, conversely, to reposition the 2D fluoroscopic image information in 3D images, it is necessary to determine the acquisition parameters of the 2D fluoroscopic images placed on the patient's body region to be examined. This is the case, for example, in U.S. Pat. No. 6,359,960 that describes a method for automatically determining the three-dimensional coordinates of markers without the intervention of a user. The method comprises detecting the positions of the markers in two-dimensional projections then, by an inverse projection, in determining the coordinates of the markers identified in three dimensions, the geometry of the projections being known.
These methods for determining the three-dimensional coordinates of markers positioned on the patient make it possible to guide surgical instruments during an operation and/or to allow merging of images, for instance projection of 3D images into 2D radiographic images or vice versa; These methods nevertheless assume that the patient remains perfectly stationary when the three-dimensional coordinates of the markers are being determined. It is very common for the patient to move during the procedure, however, leading to inaccuracy in the projections of the 3D images into the radiographic images, or vice versa, which may lead to an interpretation error of the visualized images.